Clonidine, a common antihypertensive agent, was recently shown to inhibit antigen-induced wheal and flare reactions, as well as allergen-evoked bronchoconstriction in allergic patients. In this study we investigated the in vitro anti-allergic properties of clonidine. This α2 adrenoceptor agonist inhibited, in a dose-dependent fashion, basophil histamine secretion elicited by anti-IgE and f-met peptide; it also blocked, though to a lesser extent, the calcium ionophore A23187-induced histamine release. H2 receptor stimulation seems to be responsible for the reduction in histamine release observed, since it can be antagonized by cimetidine, but not by α1, α2 and H1 blockers.
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